Magnetic tape cartridges provide a means to store data on magnetic tape to be saved and read back at a subsequent time. A magnetic tape drive writes the data to magnetic tape, typically as a set of parallel tracks, and subsequently a magnetic tape drive reads back the data. To read back the data, a magnetic tape drive typically comprises parallel read heads to read each of the parallel tracks, a drive system for moving a magnetic tape with respect to the read heads such that the read heads may detect magnetic signals on the magnetic tape, and a read channel for digitally sampling magnetic signals detected by the read heads, providing digital samples of the magnetic signals. The digital samples are then decoded into data bits, and the data bits from the parallel tracks are combined into the data that was saved. Magnetic tapes may be interchanged between tape drives, such that a magnetic tape written on one tape drive will be read by another tape drive. Variation in the response of the read heads to the variously written magnetic tapes may result in unacceptably poor read back of the recorded signals.
The read channel typically employs a number of elements to provide an acceptable readback of the recorded signals, for example comprising: an analog to digital converter (ADC) to provide asynchronous digital samples of the input signal; an equalizer to compensate for the change in the signal due to the magnetic recording properties of the write head, the magnetic tape, and the read head; a mid linear filter to obtain, additionally to the digital samples, mid-sampling-time instant values from which the asynchronous digital samples are derived; an interpolator to convert the asynchronous digital samples to synchronous digital samples; a gain control to adjust the amplitude of the synchronous digital samples; and a data detector. The values of the digital samples are typically not values of ideal read signals. In fact, they may vary considerably from the ideal values. In some instances, such as with respect to the gain control and the interpolator, error signals are fed back to allow adjustment of the parameters of those elements.
The error signals to adjust the parameters of elements such as the gain control and the interpolator, are typically derived directly from the digital samples after equalization, interpolation and gain have been applied. For example, a slicer may be provided to relate the synchronous digital samples to the closest ideal values and to detect errors between the synchronous digital samples and the closest ideal values. Those errors comprise error signals which are fed back to allow adjustment of the parameters of those elements. However, since the values of the synchronous digital samples typically vary from values of ideal read signals, and those digital samples may represent recorded signals that are substantially different than the closest ideal values (such as a digital sample of value “+1.05” may represent an intended ideal value of “+2”, but is closest to an ideal value of “+1”), the errors used to make the adjustments to the parameters of various elements may be inaccurate and result in a “noisy” feedback.